Heat treatment for magnesium alloys



Patented Jan. 7, 1947 HEAT TREATMENT FOR BIAGNESIUM ALLOYS James F. Simpson, Old Greenwich, Conn., assignor to American Cyanamid Company,

New

York, N. Y., a corporation of Maine No Drawing. Application October 14, 1944, Serial No. 558,741

11 Claims. (Cl. 148-21.3)

The present invention relates to the heat treatment of magnesium base alloys and to a fused bath therefor.

Heretofore magnesium base alloys have only been heat treated to improve their mechanical properties in a circulating gas furnace. Treatment therein is hazardous in that there is danger of fire by reason of the material treated. It has been proposed to introduce sulfur dioxide into the atmosphere of the furnace to prevent burning of the magnesium at treating temperatures but leakage of S02 is a nuisance and health hazard.

The principal object of the invention is to provide a method and means of heat treating magnesium base alloys which will produce mechanical properties in the alloys treated as good or better than those now obtained by treatment in the usual circulating air furnace, which will have no fire hazard, which will be inert chemically to the alloy, which will be cheap, and otherwise desirable.

To this end the invention contemplates the heat treating of magnesium base alloys in a molten bath comprising KCNO in predominant quantity. By this expression is meant KCNO alone or in admixture with minor quantities of such salts as KCl, NaCl, KCN, NaCN, NaCNO, MgClz, K2003, or the like. The exact composition of the bath whether of KCNO alone or in admixture with one or more of the above salts will in some measure depend upon the temperature required for the desired heat treatment as obviously no treatment in such baths may be had prior to their melting point which varies considerably. For instance, baths in which magnesium base alloys may be satisfactorily heat treated are given below together with their melting points:

Melting Bath composition I point l KCNO 100% up to a temperature just short of substantial decomposition. While the temperature and time of treatment will vary slightly dependent upon the actual composition and mass of the magnesium stantially eutectic mixtures the proportions may be reasonably varied when slightly higher temperatures are desired. Such baths may be used at temperatures varying from their melting point base alloy yet this is a minor variation as in all cases it has been found that the temperature and time of treatment is substantially that now used for the same alloys in circulating air furnaces.

The usual solution heat treatment of magnesium base alloys in air type heating furnaces involves generally heating the work to from 630-760 F., holding the piece at one or more places in the temperature scale for from 5-20 hours followed by air quenching. The same conditions have been found to be satisfactory in the heat treatment of such alloys in fused baths of the present invention.

The following alloys and their treatment are typical of practice under the present invention.

The specifications of mechanical properties for a cast magnesium base alloy referred to herein as alloy A and consisting of are as follows:

Mechanical properties Soluton Solution Condition As cast preclp1- treated Yield strength .p. s. L. 10, 000 10,000 16, 000 Tensile strength (lo 24, 000 32, 000 34, 000 Elongation, per cent in 2 l 4 6 2 The prevalent recommended solution heat treating and subsequent precipitation cycles used in conventional circulating air type heat treating furnace are as follows:

Solution treatment Precipitation treatment 4. Hold at 350 F. for 16 hours; 5. Air cool.

Test bars of alloy A were solution heat treated in a bath comprising substantially 87% potasduplicate bars treated in a conventional air furnace according to the above prevalent heat treating cycles. The solution heat treated bars were precipitation heat treated in an ordinary air oven with no chemical correctives in the atmosphere. The mechanical properties of test bars of alloy A so treated are indicated below:

Table I ALLOY A SOLUTION AND PRECIPITATION TREATED Permanent Yield Tensile lgg Hard- Bar No. Treatment strength, strength, ness,

p. s. 1. p. s. 1. cent Bnnell in 2 lengths Specifica- 16,000 34,000 3 min.

tions. mm. min.

19, 200 41, 200 6. 5 68 18, 750 36, 550 4 68 10, 850 41, 650 6. 5 67 19, 000 37, 000 5 69 18, 600 41, 400 7 68 19, 000 39, 000 5. 5 68 18,240 37,920 4 7 19, 500 41, 650 5. 5 70 18,820 30, 230 4 71 19, 500 41, 100 6 67 19, 230 34, 810 3. 5 68 v 20, 170 36, 650 4 03 CH 272,." Air furnace"... 17,220 33,460 3 70 CH 282. Bath 22, 220 38, 750 4 72 CH 282. Air furnace..." 16, 800 35, 470 4 74 SOLUTION TREATED ONLY Specifica- 10,000 32,000 6 tions. min. IIllIl.

CH 302 1. Bath .1 17, 975 38, 150 11. 5 CH 312A... do 13,225 37,000 10.5 13, 475 38, 225 12. 5 13, 350 35, 900 9. 5 14, 175 37,050 10 14, 125 37, 350 10. 5

1 Denotes edge inclusion.

Another slightly different magnesium base alloy hereinafter referred to as alloy B, composed as follows:

Materials other than the above and magnesium .30 maximum Magnesium Remainder was heated in a bath of the present invention according to the prevalent heat treating cycle currently used in the conventional circulating air type furnace, as follows:

Solution treatment 1. Heat to and hold at 760 i 5" F. for hours; 2. Air cool.

Precipitation treatment 3. Heat to 350 F. and hold for 12 hours; 4. Air cool.

The results of the treatment of samples of alloy B according to the above is shown below as compared with the standard specification of mechanical characteristics for the conventionally treated alloy:

Table II ALLOY B SOLUTION AND PRECIPITATION TREATED Permanent Yield Tensile Bar No. Treatment strength, strength, .s.i. i. a p 2 lengths Specifications 18, 000 34, 000 1 'B. 0 23,800 35, 900 1 B. C 24, 000 36, 250 l B. C 23, 450 40, 600 1. 5 C. C 26, 35, 200 l C. C 25, 900 34, 800 l O. C 23, 750 30, 850 1 C. C 23, 700 37, 700 l SOLUTION TREATED ONLY Specifications 10, 000 32, 000 a 0.0.181 Bath 15, 900 33, 500 e In all cases of treated alloys A and B neither the solution heat treatment nor the precipitation treatment detrirnentally affected the mechanical properties nor the surface condition of the alloys.

It was found that such baths actually act as an insulator against the burning of the magnesium base alloys even when severely overheated.

Teatment in a fused salt bath is particularly advantageous over air furnace heating in that the work piece can be suspended from wires, thus eliminating the necessity of heating the nonproductive portion of the charge such as trucks, baskets and the like required in the standard air type heating furnaces.

Any bath material adhering to the Work after tr tm nt m y b readily Wash d off in Water.

Baths of this type lose only a very small part of the content through vaporization as it has been found that only 2.4% by weight was lost through such cause after 40 hours of continuous operation at 750 F. In addition, any slow increments in decomposition products of the bath up to a concentration of more than 30% is completely soluble in the bath at the treating temperature. This minimizes sludge formation and consequent detrimental effects on the heating pot and workpiece due to localized overheating. The presence of the decomposition product or products has no apparent efiect on either the mechanical properties nor the surface condition of the alloys treated.

In all cases where the treated alloys were etched for five seconds in a standard glycol etchant it has'been found that there was vast improvement in distribution of the component parts of the alloy as compared with an untreated piece.

While the invention has been described with particular reference to specific embodiments, yet it is to be understood that the invention is not to be limited thereto, but is to be construed broadly and restricted solely by the scope of the appended claims.

What is claimed:

1. A method of heat treating a magnesium base alloy work piece which includes the steps of immersing the same in a molten bath predominately ofKCNO at a solution heat treating temperature until the distribution of the alloy components has been improved, removing the work piece from the bath and air quenching the same.

2. A method of heat treating a magnesium base alloy work piece which includes the steps of immersing the same in a molten bath predominately of KCNO and also containing KCl at a solution heat treating temperature until the distribution oft-the alloy components has been improved, removing the work piece from the bath and air quenching the same.

3. The method-of claim 2 in which the bath contains substantially 87% KCNO and substantially 13% F101. I

4. The method of claim 2 in which the bath temperature is from 6306 i F. and the time is substantially 5 hours. a

5. The method of claim 2 in which the bath temperature is from 630-640" F. and the time is substantially 5 hours, and thereafter raising the bath temperature to from 710-720" R, holding that temperature for substantially 13 hours, removing the work piece from the bath and air quenching.

6. The method of claim 2 in which the bath temperature is from 630-6i0 F. and the time is substantially 5 hours, and thereafter raising the bath temperature to from 710-720 F., holding that temperature for substantially 13 hours, removing the work piece and air quenching, heating the work piece in air to substantially 350 F., holding the same for substantially 16 hours, removing the work piece and air quenching.

'7. A method of heat treating a magnesium base alloy Work piece which includes the steps of immersing the same in a molten bath containing substantially 8'1 KCNO and sustantially 13% KCl at from Till-720 F. for 13 hours, removing the work piece, air quenching, heating the work piece in air for substantially 16 hours at substantially 350 F., removing the work piece and air quenching.

6 8. The method of claim 6 in which the alloy is of substantially the following composition:

Aluminum 5.3 -6.7 Zinc 2.5 -3.5 Manganese .15 maximum Silicon .5 maximum Copper .05 maximum Metals other than the above and magnesium .3 maximum Magnesium Remainder Aluminum 8.3 -9.'7 Zinc 1.7 --2.5 Manganese".- 1.0 maximum Silicon .50 maximum .05 maximum .03 maximum Copper Nickel Materials other than the above and magnesium .30 maximum Magnesium Remainder and is solution treated at substantially 760 F. for about 20 hours, removed, air cooled and then heated for about 12 hours, at substantially 350 F. in air, removing the work piece and air quenching.

JAMES F, SIMPSON. 

